Seyfert
Galaxies

Almost all Seyfert galaxies are
spiral galaxies and have been among the most intensively studied
objects in astronomy, primarily because they are thought to be nearby,
low-luminosity versions of the same phenomenon observed in quasars.
A massive black hole in the nucleus of a galaxy, accreting gas from
its surrounding environment, is thought to power all these objects.
Of course, we do not see the black hole itself, but the UV continuum
radiation is generally presumed to be thermal emission from the
hot gas that forms an accretion disk surrounding the black hole.

Seyfert galaxies, which amount to 10% of all galaxies, produce
nuclei spectral line emissions from highly ionized gas.
These galaxies are named after Carl Keenan Seyfert>, who identified
this galaxy class in 1943. Seyferts are a subclass of active galactic
nuclei and it is thought they contain extremely large black holes.

This montage uses ground-based wide-field
images to exhibit the variety of types and environments in which Seyfert
are found. Of these, IC 4329A, NGC 3516, and Markarian 279 are type
1 Seyferts, NGC 3786 is an intermediate "type 1.5" nucleus",
and NGC 5728 and NGC 7674 are type 2 objects.

This might resemble a fried egg you've
had for breakfast, but it's actually much larger. In fact, ringed
by blue-tinted star forming regions and faintly visible spiral arms,
the yolk-yellow center of this face-on spiral galaxy, NGC 7742, is
about 3,000 light-years across.

Pronunciation: (si-'furt,
se-'-), [key] Astron. one of a group of spiral galaxies with compact,
bright nuclei having characteristically broad emission lines suggestive
of very hot gases in violent motion at the center.

Seyfert galaxies were
originally noted as having unusually bright, compact (starlike) nuclei.
The surroundings of this brilliant nucleus can take a variety of forms,
perhaps carrying clues to how the central engine is fed or triggered.

Seyfert galaxies were
originally noted for the strength and broadening of their emission
lines, and as a class were later characterized by the high ionization
states of many of the atomic and ionized species producing these lines.

What's going on here?
In fact, how many different things are going on here? NGC 1275 has
been called, at various times and not always by different astronomers,
a Seyfert galaxy, radio galaxy, and even blazar...

While many active nuclei
announce their presence from a tiny, intense source of blue and ultraviolet
light, some are hidden from our view by intervening dust - either
surrounding the central engine or happening to lie somewhere else
in front of it.

Measurements of the polarization
of the light near the nucleus of NGC 1068, a nearby and prototypical
type 2 Seyfert, provided strong evidence that it actually contain
a type 1 nucleus which is blocked from our direct view by an obscuring
ring or torus of material

A crucial development
in understanding Seyfert galaxies came with the recognition that at
least some type 2 Seyfert nuclei are in fact type 1 objects for which
our view of the innermost region is blocked by a dust- and gas-rich
obscuring torus.

When examined spectroscopically,
galaxies are generally found to have absorption features similar to
those found in stars. This is what one would expect for the integrated
light of a population of normal stars. However, in 1943 Carl Seyfert
identified a class of spiral galaxies (later given the name ``Seyfert''
galaxies) which exhibit broad forbidden emission lines of [O II],
[O III], [N II], [Ne III], [S II], and [S III], coming from unusually
bright central regions.

Active galaxies are thought to be powered
by the accretion of gas onto a central massive black hole. Seyfert
galaxiesthe most common examples of nearby active galaxiesare
separated into two classes based on their emission line widths.

We suggest that there exists an evolutionary
sequence from Seyfert 2 to Seyfert 1 galaxies. We review observations
which support this suggestion and the idea that the main parameter
is the dust content in and around the Broad Line Region (BLR).

Bulge stellar velocity
dispersions, , in Seyfert 1 galaxies are combined with the M relation
to validate reverberation mapping as a means of determining nuclear
black hole masses. Seyfert galaxies follow the same relation as non-active
galaxies, indicating that reverberation mapping is consistent with
other methods.

The ROSAT Ultradeep HRI survey in the Lockman
Hole contains a complete sample of 91 X-ray sources with fluxes in
the 0.52 keV band larger than 1.2×10-15 erg cm-2 s-1,
where over 75% of the sources are quasars or Seyfert galaxies.

In 1943, astronomer Carl Seyfert noticed
that certain nearby spiral galaxies have very bright, pinpoint nuclei.
Spectra of these galaxies, now named Seyfert galaxies, showed that
they have unusual spectra with very strong, often broad, emission
lines.

Using a technique that peeks over obscuring
rings of dust and gas and into the hearts of distant galaxies, a researcher
has found evidence suggesting that as many as half of the bright,
active galaxies known as Seyfert 2 galaxies may have significantly
less active central black holes.